This invention relates to a driving circuit for a DC motor including a brushless motor and a motor with a brush.
For example, a brushless IC motor and a motor with a brush are widely used as a DC motor. The brushless IC motor is driven by detecting the position of a rotor by hall elements and controlling switching transistors by the detected signals and thus sequentially switching stator coils to be excited. A motor with a brush is adapted to switch excitation currents into rotor coils according to the state of electric contacting between the brush and the commutators.
In this motor with a brush, DC voltage continues to be supplied to an exciting coil during a period of time required from the contacting of the brush with one commutator to the contacting of the brush with the next commutator. Therefore, a period of time for the DC voltage with a brush to be supplied to one exciting coil during the rotation of the rotor is very short. Further, during this short period of time, the exciting current does not adequately rise due to the presence of an inductance of the exciting coil. Still further, a counter electromotive force is generated in the exciting coil according to the rotation of the rotor, in which case only a part of voltage of a DC power source is applied between both the terminals of the exciting coils because of that the counter electromotive force is subtracted from the voltage applied across the terminals of the exciting coil. As a result, a smaller current flows through the exciting coil. If a greater driving force is required, it is necessary that the voltage to be applied across the exciting coil be set at a higher level. Since at the time of starting of the motor the rotor is at rest, voltage is applied across the exciting coil for a prolonged period of time and no counter electromotive force is generated in the exciting coil. If, at such a state, a higher voltage is applied across the coil, an excessive current flows through the exciting coil at the time of starting of the motor, causing a possible damage to a driving circuit and power supply circuit.
As the brushless IC motor, use is often made of three-phase brushless motor. In such three-phase brushless motor, one exciting coil is excited during a one-third cycle period and thus a voltage is applied across the exciting coil during a long period compared with that of the motor with the brush. If, therefore, excessive current flows across the exciting coil at the time of starting of the motor, the driving circuit and power supply circuit are liable to be damaged.